The radio spectrum is a finite resource expected to accommodate ever increasing numbers of communication nodes whether in a commercial environment or in a military environment. Tactical military and commercial applications require self-organizing, wireless networks that can operate in dynamic environments and provide peer-to-peer communications. For multiplexed communication systems, the communication system typically consists of many communication nodes that require active service over a single communication channel. As a result, a variety of multiple access schemes have been devised that allow multiple users to share the same communication channel. For example, communication systems have been developed to provide communication between many communication nodes for brief intervals of time using the same communication channel. Such a multiple access scheme is known as Time Division Multiple Access (TDMA). A communication system that employs TDMA is referred to herein as a TDMA system or a TDMA network.
A TDMA communication system can be used in conjunction with other types of multiple access systems including frequency division multiple access and code division multiple access systems. For example, frequency hopping using a frequency synthesizer to “hop” the signal from one frequency to the next can be used in a TDMA communication system to improve performance characteristics including providing a low probability of intercept. The communication nodes are synchronized to follow a frequency hopping code thereby ensuring that the communication nodes are on the same frequency at the same time. The hopping pattern may be assigned to each node in its identification code. As a result, it is difficult for a communication node that does not know the frequency hopping code to synchronize with the other communication nodes.
An advantageous characteristic of a TDMA system is the ability of neighboring nodes to transmit without interference. In a TDMA system, each carrier frequency is divided into repeating frames. The frames are subdivided into a plurality of time slots. A communication node within the TDMA system is assigned one or more time slots for transmitting and/or for receiving a communication signal. Each communication node is assigned particular time slots in a continuum of recurrent frames for transmission of its bursts and for reception of its own bursts and the bursts of other nodes. The time slots are designed to be non-overlapping when the various nodes' signals arrive at the receiver. A base radio may manage all of its assigned communication nodes by keeping all of them synchronized and allowing each node to communicate at a known, or deterministic, time. Alternatively, the communication nodes may use a self-organizing system to allocate time slots and to maintain synchronization with each other. For example, the Unifying Slot Assignment Protocol (USAP), which is disclosed in U.S. Pat. No. 5,719,868, provides a protocol for maintaining such a self-organizing communication system.
With the growth in the number of communication nodes, the need to quickly form and merge networks as nodes move into a neighborhood has increased. Mobile, multi-hop, broadcast packet radio networks provide rapid and convenient deployment, self organization, mobility, and survivability. In this type of network, a transmission from one node is broadcast to all nodes in its “neighborhood”. Ultra-high frequency (UHF) systems generally have a neighborhood defined by nodes within line of sight of the transmitting node. For illustration, FIG. 1 depicts an overlapping neighborhood system 10 that includes a first neighborhood 18, a second neighborhood 20, and a third neighborhood 22. The first neighborhood 21 includes nodes 1-5 and f that are within line of sight of node 3. The second neighborhood 22 includes nodes 2, 3, 5, a, c, d, and f that are within line of sight of node f. The third neighborhood 23 includes nodes a, b, c, d, e, f, and g that are within line of sight of node c. All of the nodes in a neighborhood, however, may not be in communication with the other nodes in the neighborhood. For example, two networks encompass the communication nodes of system 10. A first network 12 includes nodes 1-5 that are in communication with each other, and a second network 14 includes nodes a, b, c, d, e, f, and g that are in communication with each other. Multiple hops may be required, for example, to provide communication between node f and node e. The second neighborhood 20, however, includes nodes 2, 3, and 5 that are not currently in communication with node f as indicated by the broken lines. The ability to merge network 12 with network 14 in a time and a power efficient manner is a key capability of a TDMA system.
To avoid interference between communication nodes, a TDMA system requires good clock synchronization between the communication nodes. Timing acquisition is a process for synchronizing the receiver's clock with the transmitter's clock so that the receiver can determine the boundary between two transmitted symbols. In general, timing acquisition is performed by sending a preamble before information bits in a TDMA frame. Additionally, each communication signal transmitted in a time slot includes a predefined synchronization word. The synchronization word must be matched in order to validate the communication that follows. If no match is achieved, the information can not be processed from the communication signal. By identifying the location of the synchronization word in a received signal, a node maintains synchronization with the other nodes in the network. This scheme requires a reasonably accurate clock timing recovery before frame synchronization (or any form of communication) can take place. Any false or missed detection of the synchronization pattern results in a loss of the information in the data frame. This poses a particular problem in a TDMA system operating in a dynamic environment in which nodes move in and out of neighborhoods. The new node must synchronize with the TDMA system before communication between the new node and the TDMA system can be accomplished.
For example, to establish synchronization with a new node to a neighborhood such as the node f, a frame synchronization pattern is periodically inserted into the data stream by a transmitter within the first network 12. In an example protocol, once every few seconds a network synchronization time slot is allocated to transmit the frame synchronization pattern. A communication node within the first network 12 may pseudo randomly transmit the network synchronization information. The remaining communication nodes in the first network 12 listen. The node f may similarly transmit synchronization information for the second network 14. Based on statistical theory, the first network 12 and the second network 14 can be merged within an expected time delay assuming that both the first network 12 and the second network 14 are using the same TDMA protocol, for example, based on one of the nodes a, c, d, or f receiving the network synchronization information from a node (for example, nodes 2, 3, or 5) included in the first network 12 or a node included in the first network 12 receives the synchronization information from one of the nodes a, c, d, or f.
A common and accurate representation of time is also assumed in determining the likelihood of successful synchronization and the time delay associated with merging the first network 12 with the second network 14. If, however, the clock of the second network 14 is not synchronized with sufficient accuracy to the clock of the first network 12, significant time delays (possibly on the order of hours or days) can result before synchronization between the first network 12 and the second network 14 is completed. Significant delays may result due to clock inaccuracies of only 100 μs. What is needed, therefore, is a system and a method that improves the merging of networks within a multi-hop neighborhood into a single network. What is further needed is a system and a method that reduce the waste of communication resources.